Telephone transmitter



|-I c. PYE

TELEPHONE TRANSMITTER Y Filed Sept. 16, 1955 March 7, 1939.

2 Sheets-Sheet 1' CARSON INVENTOR.,.

-' HAROLD C.PYE

ATTORNEY.

March 7-, 1939.

FIG.5

FIG.6

PIC-3.7

FIG.8

Flea;

NOISE UNITS H, c. PYE

TELEPHONE TRANSMITTER FIG4A. F'IGAB. FIG4Q DECIBELS c 6 1 15mm; mom VERTICAL POSITION m means so 70 so 50 40 30 20 0 OHMS 3 40 LU 30 m T G 20 LL! 0 10 FREQUENCY CYCLES PER SECOND -50 m 40 g 30 Q 20 TIME IN MINUTES ARTlFICIAL AGING OPERATIONS HAROLD C PYE A TTORNEY.

ARBITRARY Pennies ar. 7, 19s

'l'lE STATES TELEPHONE 'mANsm'r'r Harold C. Pye, Oak Park, IlL, assignor to as sociated Electric Laboratories, Inc, @hicago, 111., a corporation of Delaware This invention relates-in general to telephone transmitters but more particularly to transmitters of the type employing carbon granulated material in the microphone cell and the broad object of the invention is to provide a transmitting unit particularly adapted for use in the handset type of instrument employed with the cradle type of telephone.

The principal object of the invention is to design a transmitter unit that will operate to transmit speech in; a. highly satisfactory manner regardless of the position-in which the unit is h d by the handset when in use.

V ore proceeding'witha detailed description of the invention it may be well to generally review the general characteristics of previous types of transmitter units and their particular defects or iailings which is. the general object of the transmitter concernedin the invention to over-.

come. Y

Since the initial introduction of the handset type of telephone unit, (transmitter and receiver mounted as a unit connected by a handle), par-' ticularly close contact has been maintained byengineers and others to note the performance of these units by tests and various observations and of those which have been in service for different lengths of time under numerous conditions of use. These tests and observations have made .7

desirable properties in a handset transmitter are evident the desirable properties of an ideal handset transmitter, and in addition have pointed out the faults and weak points of the existing ones. In addition to testing and observing handsets in actual service here has been devised methods and means simulating the actual conditions of service by means of tasting machines. These testing machines are essential in order to give accelerated ageing tests, position performance tests, transmitter response measurements, and many other tests that are necessary to accurately rate the performance of many proposed designs, in a reasonable length of time.

In order to make the. problem and therefore the general object of the invention clear it may be current supply on short length of lines without 7 developing carbon. noise.

(c) A handset-transmitter which can replace 55 any fixed stationary transmitter type of telewell to set down the desirable properties of an;

phone without any deterioration of performance.

(d) A handset transmitter whose resistance is low enough to operate the central oilice signalling apparatus satisfactorily on all types of switching equipment. The resistance of the unit must meet with the foregoing requirement regardless of the position of the transmitter in which it is held,

atmospheric conditions, or the length of time in which it has been in service.

(e) A transmitter unit in which the fidelity of response is as good as the present state of the art of electro-acoustical design willpermit, without sacrifice of transmitting volume efliciency.

(f) A transmitter unit in which the response is independent of' the position in which it is held while in use.

' (g) A transmitter in which the volume emciency or sensitivity does not change either gradually or periodically while in actual service.

(h) A unit which is independent of atmospheric conditions such as humidity, temperature, and pressure. Moisture from the breath condensing on the transmitter parts should not affect them.

becoming noticeably noisy or declining intransmitting eficiency. Y

' It will be obvious that some of the foregoing antagonistic to each other. A transmitter, as is will known, depends for its performance upon the movement of the diaphragm causing a resistance change in the microphone cell proportional to the movement. A given movement of the diaphragm will cause acertain percent change of the micro- V (i) Atransmitter unit which. is able to give" several years of normal telephone service without phone resistance. It is therefore obvious that g the higher the microphone cell resistance, the greater will be the common battery line resistance change and in a circuit the more sensitive will be the transmitter. Contrary to this, however; the higher the transmitter resistance, thehigher will be the voltage across the microphone cell. The heat generated at the points of contact of the carbon granules in the cell is proportional to the square of the voltage between them This heating causes thermal agitation of the granules and if great enough will be heard as a frying or,

sizzling noise in the receiver.. In. other words,

the higher the transmitter resistance the greater the heat generated at the points of contacts of the carbon granules. The onlyknown solution of this apparent dilemma is to so design the transmitter that the motion of the diaphragm causes a large percentage of change of a and to make the resistance low enough to handle ciency and are therefore entirely unsuited for ordinary telephone purposes. .The fidelity of transmitters can, however, be much improved by the proper balancing of the resonant frequency of the diaphragm, the acoustical chambers in front and in back of the diaphragm together with the proper amount of damping, without a sacri fice of volume efliciency There have been many designs proposed tending to reduce the variation of performance of the'transmitter with different positions of the These range all the way from mounthandset. ing the transmitter atsome advantageous angle in the handset, to various shaped electrodes and microphonecells. Placing the transmitter at some angle is only a compromise as there" will be some position in which, the performance is poor even if this is farther removed from the normal position. Electrodes have been formed as cones of various angles, hemispheres, cylinders, and many other geometrical forms, with the purpose of maintaining contact between the granules and the electrodes in all of the different positions. Other designs involve immersing the electrodes in the granules so that contact is maintained between them in all the different positions. This is the type most commonly used commercially in transmitters at the present time.

It may be of interest here to analyze the action taking place in a. microphone cell with the purpose of showing what is necessary to obtain all position" performance, that is, correct oper ation of a handset transmitter when placed in use at any desired angle. In the quiescent state the carbon granules exert a pressure between each other and the electrodes due to the force of gravity. The carbon granules at the bottom of the cell (that is, in a direction toward the earth) will be compressed greatest,'while those at the top of the cell least, and the pressure between'the topand bottom will be some intermediate amount. The first requirement for all position" performance is that the cell beso shaped that this quiescent compression be the same regardless of its position, for the resistance of the cell depends upon the compression of the granules. This. could be quite simply accomplished by means of some approximately spherical form for the cell with the electrodes in the center if it were not for'the fact that the gran-' ular carbon does not behave like a fluid. In a fluid the pressure in all directions at a given point is the same. Because of the friction between the carbon granules the pressure in a horizontal direction is much less than in the verti-.

cal direction. If two oppositely disposed plane parallel electrodes are placed in the center of a spherical cell of granules the resistance of the cell will be much greater when the electrode surfaces are horizontal than when they are in vertical position, even if the cell diameter is large with respect'to the electrodes diameter. This is so because the pressure is not transmitted horizontally between the electrodes as well as it is vertically. It is in this respect that the immersed electrode types of transmitter cells failed to fully accomplish their .purpose.

The movement of the diaphragm under the action of sound waves compresses the granules moreor less than they were compressed in the quiescent state, depending upon the direction of motion. This pressure aspointed out is not transmitted equally well in all directions through the granules, but is greater inthe direction of motion, becoming less as the angle tothis direction is increased until at right angles it is at a minimum. It is for this reason that many of the variously shaped electrodes are not successful. With the electrodes in the form of a cone the volume efliciency is less with a sharp cone than with a blunt one because less of the diaphragm movement is effective in compressing the granules. In the hemispherical or other curved surface electrodes the movement of the diaphragm is most effective'in compressing the granules near the top of thehemisphere where the surface is most nearly at right angles to the direction of motion or compression of the granules. The sides of the hemisphere are almost parallel to the direction of motion and are therefore almost inactive. This inactive portion of the cell acts as an electrical shunt resistance on the active portion of the granules.

There has been considerable research made to find the ideal microphonic material. Many metals, minerals, and' combinations of materials have been tried but it seems that carbon most nearly meets all of the conditions required. It is hard, has a. low density compared .to most metals, cannot be fused, does not corrode in air, can be made with a high degree of purity due to its inert nature, and above all, its microphonic properties are ideally suited for transmitters. Processes have been developed to manufacture both granular and electrode carbon with uniform properties at. a reasonable cost. It is therefore possible to take advantage of its high microphonic efliciency for both granules and'electrode materials. The use of metal electrodes in a transmitter cell either solid or plated with various. alloys loses part of the microphone ad-- vantage of carbon at the electrode. surfaces and depends entirely upon the microphonic action between the granules. Metal surfaces are quite likely to be soft and easily abraded by movement of the carbon granules.

With the foregoing objects in mind a transmitter unit has been designed according to this invention which most nearly meets all the conditions of an ideal handset transmitter as above enumerated and without employing expensive materials or construction methods in its quantity manufacture.

A preferred embodiment of the novel transmitter unit according to the invention is illustfated in the accompanying two sheets of drawings in which Fig. 1 shows a side cross sectional view through the center of the transmitter unit; Fig. 2 is a rear view of the transmitter; Fig. 3 is an exploded perspective view of theelements comprising the microphone cell andthe diaphragm; Figs. 4, 4A, 4B, 40 and-4D illustrate various shaped electrodes that maybe used; Figs. 5 to 9 illustrate a number of graphs diagrammatically indicating performance characteristics of. the transmitter unit made according to the invention.

Referring now more particularlyto Fig.v i of The form is's'uch that it can be used interchangeably with either the handset type of transmitter unit or the rigid desk stand type.

used with the unit are not illustrated here. The

housing l is cup-shaped and is arranged to support the various members and parts of .the complete transmitter unit. An aluminum face plate 2 having a number of openings 3 arranged in a The various shaped mouthpieces and supports which may be circular pattern for the purpose of permitting the speech waves to reach the diaphragm, is placed in the flange 6 of housing #1. A thin edge 1 around the flange 6 is then spun over on the edge of face plate 2 to hold it firmly in place in the housing i. The (groove 5 cut in the periphery of face plate 2 enables a sort of springing action to be applied to plate 2 when the edge d is pressed down and spun over on the face plate and a A tighter fit to be brought about.

The moving system comprising the diaphragm is made of extremely light material such as thin directly on the flange of diaphragm l to the inside of the housing l. Behind the main diaphragm '1 lies a secondary diaphragm E9 in the shape of a conical ring having an opening in the center thereof. The secondary diaphragm it has a' dome shaped cup H inserted through its opening which is provided with a flanged portion l4 resting against the edge of the opening. The

complete assembly comprising the diaphragm l,

secondary diaphragm Ill, and the dome shaped cup ii are then drawn together under tension to the position indicated in Fig. 1 and secured between the top of the dome H and the center of the'main diaphragm 1 by means of the eyelet or rivet 112; thus forming a diaphragm assembly,

l which is very stiff over the center conical portion and extremely flexible around the outer edge flange of the diaphragm l, the outer edge of the diaphragm I being securely clamped as has been pointed out. The eyelet I2 has an opening l 3, the'purpose of which will be later on described.

.By-"utilizing the eyelet and rivet 12 to hold the assembly together in the center of the diaphragm '1 it is not necessary to provide additional means for'rigidly securing the secondary diaphragm I0 at its periphery to the main diaphragm I, or at the inner "edge of the opening of this secondary This condiaphragm to the flange ll of cup I l. struction insures that the diaphragm moves as a whole similar to a piston and will not-vibrate in sections asa flat plate does when driven by sound waves of various frequencies. .Theforegoing assembly is in the nature of a truss diaphragm. A ring shaped electrode [5 constructed of solid shaped cupll by turning up-or spinning over the edge of the flange [4 on the electrode, thereby securely holding the electrode in its set position. The moving electrode I5 is thereby rigidly fasdiaphragm Tl are clamped by the face plate '2 carbon-is fastened tothe flange I4 of the dome v tened to the diaphragm system and moves with it as a unit. Opposite the moving electrode is a fixed ring shaped carbon electrode it of the same shape as the moving electrode i5. This is mounted in the spun over edge of the flange l9 ,of another dome shaped cup it which cup is" rigidly fastened to the transmitter housing 1 by means of the nut or threaded ring 23. The cup E8 is electrically insulated from the housing l by means of the ring shaped insulating washer 2i and the insulating cylindrical ring 2d on the shank of the dome shaped cup l3. Between the outer edges of the carbon ring electrodes i5 and it there is cemented a flexible paper bellows El of circular shape as will be seen more clearly in .Fig. 3. This paperbellows ll confines the granular carbonin the cell which is formed between the two cups H and I3 and the electrodes l5 and it. This bellows may be attached by means of a coating of varnish or-other adhesive to the edges of the electrodes. As a further function of the paper bellows ll it provides a means so that as the moving system of the diaphragm is vibrated air is forced through the. folds of the paper bellows ll between the outside'of the microphone cell and the chamber in back of the diaphragm, thus slightly damping or controlling the vibrations of the diaphragm. The resonance chamher in front of the diaphragm l and moisture proof covering 9 and behind the face plate 2 provides a further damping means for the moving v system. The inner surfaces of the dome shaped cups l 3 and l 8 in contact with the carbon granules are covered with a suitable insulating enamel as indicated in Fig. 3 at 3i and 32 so that the current flow in the microphone cell is confined to the carbon electrodes l5 and It. For the purpose of illustrating the individual parts of the microphone cell more clearly the carbon granular material which almost completely fills up the space between the electrodes and the dome shaped cups H and it, has not been shown in the drawings.

At the rear end of the dome shaped member it there is provided an opening whereby the micro- 3!? is threaded into the opening and then the head of the plug is sheared or twisted 01f, precluding the possibility thereafter of unauthorized tampering with the parts comprising the microphone cell.

The clamping ring 23 in addition to holding the dome shaped member l8 insulatingly clamped to the housing l directly-clamps the terminal member 27! to it. At one end the terminal member I diaphragm I and electrode 15. The other terminal member 2a placed on top of but insulated forterminal member 21 by insulating washer 22 and insulator 20 establishes electrical contact through the clamping ring 23 with the rear elecinto the housing I which is insulated therefrom by means of the insulating washers 26. Screw 25 trade Hi. It has the holding screw 25 threaded I serves as the attaching means for the outgoing .wire. The electrical circuits for the microphone cell extend from the attaching wire on screw 25. the terminal member 24,, clamping nut 23, dome shaped member l8, the electrode l6 which is held I in the flange I!) of the member I 8. through the carbon granular material filling the. cell to the other electrode l5. Then the current passes out through the flange i4 and the dome shaped member H, diaphragm 'I or secondary diaphragm ID,

to the periphery of diaphragm l, the housing I which is in contact with the edge of this diaphragm, through terminal member 21 in contact with the rear of the housing I, to screw 28 and the outgoing wire. All of the current flowing through the transmitter unit takes place over the foregoing electrical path.

The microphone cell is purposely constructed in the form described and illustrated so that pressure due to the force of gravity on the granules at the electrode surfaces is very nearly equal for any position in which the cell is placed when in use. For the reason that the pressure is not transmitted as well in a horizontal direction between flat surfaces, the electrodes 15 and [6 have been formed as narrow rings, and the horizontal direction of pressure flow is thus made very short. To assist further in the transmission of pressure between the electrodes the'inner edge of them has been beveled at a small angle. In Fig. 4

various shaped electrodeswith rounded edges and-beveling angles are indicated which may be equally as well used for this purpose. The movement of the diaphragm I under the action of sound waves impinging on the front of it compresses the carbon granules between the electrodes and also compresses the mass of granules in the cell thereby varying the resistance of the electrical current flow therethrough to electrically transmit the speech waves.

The common trend of development of telephone transmission apparatus appears to be at the present time toward a more faithful reproduction of the voice. A measure of this fidelity is the response of a transmitter with frequency variations. The range of frequencies usually transmitted over telephone circuits extend from 360 to 3500 cycles. The higher frequencies are necessary to make the voice clear and natural. An ideal transmitter would transmit all frequencies equally well. An indication of the improvements that have been made toward this direction may be had by considering that the old desk transmitters responded hardly at all above 2000 cycles and that the total variation of response between 300 and 2500 cycles was 40 decibels or more. Previous types of handset transmitters, however, responded very well up to 3500 cycles but the variation with frequency was too large, there being 30 decibels between 300 and 3500 cycles. Referring to the graph of Fig. '7 it will be noted that the improvement in the transmitter according to the invention is quite considerable. It responds up to about 4500 cycles frequency and has a total variation in the response of only about 12.5 decibels up to that frequency, thereby providing an increased range of operation and considerable better performance.

To measure the effect of changing the position of a handset upon the transmitting efiiciency requires some means of maintaining all other conditions constant while the position is changed. The well known artificial mouth equipment with the transmitter in fixed relationship to the mouth and so mounted that it can be moved to any angular position, is well adapted to this purpose. This artificial mouth equipment is described in the Bell System Technical Journal, vol. 2, No. 2, page 293. The action of the transmitter according to the invention when subjected to tests with the machine is graphically illustrated in Fig. 5 in which the various positions of the handset are indicated at the different angles from the vertical position. It will be seen from this graph that the transmitter has a total'variation of only 1.7 decibels throughout a range of different positions while former types of handset transmitters have shown a total variation of as much as 10 or more decibels for the different'positions.

Under the same tests the resistance of the transmitter microphone cell was measured with the handset position changed to various angles as indicated in Fig. 6. As shown on this graph the resistance limits for the transmitter are 29 to 3'7 ohms indicating a very small change of resistance compared with previous types of transmitters which vary from 30 to ohms with changes of position.

It is well known that breathing is a periodical variation of transmitter resistance due to the expansion of air in the microphone cell and of the cell walls by the action of heat which is generated by the passage of current through the cell.

Nearly all transmitters in use are of the sealed 1 cell type which develop breathing if left in the circuit for several minutes. This disadvantage has been eliminated in the transmitter according to the invention by providing a small air vent l3 in the eyelet I2, Figs. 1 and 3, extending from the interior of the cell to the space between the diaphragm l and the protective covering 9. The opening I3 is small enough so as to prevent escape of the carbon granules while permitting the heated air to pass through to the space. The cup shaped form of the cell walls of members II and I8 are such that the heating of them causes a slight expansion in a direction so as to lower the resistance of the carbon granules rather than to raise it as is usual in older designs. The results of these improvements are evident on the graph indicated in Fig. 8 of the transmitter resistance plotted over a period of several minutes. The older types of transmitter units when in use indicate definite increase in resistance at periodic intervals over a period of time, while the transmitter according to the invention indicates no such action whatsoever.

It is well known that packing. of the carbon granules in a microphone cell will effect a reduction of the resistance of a microphone cell due to the granules assuming a cofiguration that results in abnormal pressures between them in the quiescent state. As is clearly seen in Fig. 1 approximately half of the carbon granules of the microphone cell are carried directly in the moving system comprising diaphragm I, electrode l5, and the cup II. It is therefore seen that any movement of the diaphragm 1 moves these granules thus breaking up any packed condition and restoring the transmitter to its normal sensitive condition.

The so-called carbon noise is the chief limitation to current carrying capacity of a transmitter. As is well known a handset transmitter will receive rather rough treatment in service, such as dropping on the cradle or on a table, moving about while in use, and placing in a horizontal position on the cradle just before being used; This all tends to make the transmitter become noisy when in use. To reduce these effects to a minimum the transmitter according to the invention was designed to have a lower resistance in all positions than previous types did. In order to test the new transmitter an artificial ageing cycle was performed by a machine that simulated the condition of removing the transmitter from the cradle and raising it to the talking position and then dropping it on the cradle, the current arcades:

supply being turned on and on as it would be by the cradle switch.- The operation of dropping the handset on the cradle does the most damage.

This artificial ageing machine gives a more severe shock in this respect because the transmitter is dropped directly upon a non-resilient'anvil. The cradle switch and the handset mounting reduce the shock in actual practice. As indicated in the graph of Fig. 9, 20,000 of the foregoing arti ficially ageing operations of the transmitter oi the invention have been made; The graph indicates both the resistance oi? the carbon granules and the carbon noise level taken with a battery feed current of approximately 0.250 ampere.

From these curves it is seen that the resistance I and noise level of the transmitter reaches a fairly constant value after about 10,000 operations. The resistance and noise are less after 20,000 operations of the transmitter of the invention than older types of handset transmitters were at approximately only 1,000 operations. 20,000 ageing operations would be practically the equivalent of ten years of service with about five operations a day. This graph indicates that the ageing of the transmitter carbon granules due to shock has been elimiated to a large extent by the new design.

The use of the anti-side-tone telephonesubstation circuit efiectively prevents howling or excessive side tone'when a handset is connectedto the line. When a telephone is used on a short line loop and before the called party answers the line circuit is virtually open. Under these conditions any side tone circuitis not balanced and in some casesthe handset may tend to sing or howl. Although this does not usually take place during conversation it is nevertheless annoying. With the transmitter of'the invention as indicated in the graph of Fig. '7 the relatively fiat responseirequency curve of the transmitter greatly reduces this tendency to howl and sing. "The very light moving system of the transmitter likewise reduces the mechanical couplingbetwe'en it and the receiver in the handset. This transmitter can therefore be used on any type of circuit, side tone or anti-side tone without self-oscillations im- The transmitter can therefore be immersed in Water for a considerable length of time without any impairment in its performance.

The invention having" been described what is considered to be new and is desired to be, protected by Letters Patent will be set forth in the following'claims. i

. What is claimed is? e 1. In a transmitter, a cone-shaped diaphragm,

a dome-shaped cup secured'to the apex of said diaphragm, a conical ring clamped between the outer edge of said cup and said diaphragm, and an electrode secured tothe outer edge of said cup.

2. In a transmitter, a diaphragm comprising two conical-shaped members having their peripheries incontact, a cup memberhaving its bottom secured to' the apex of one of said conical members and its edges in contact with the other'of said from a V 5 members, and an electrode secured to the edge of said cup member.

3. In a telephonetransmitter, a microphone cell including two cup' shaped members oppositely disposed, an electrodem'ounted on the edge of each of said cup members, a flexible member conmeeting said electrodes together and enclosing said microphone cell, and a diaphragm attached to the base of one of said cup members.

4; In a telephone transmitter, a pairof oppo= sitely disposed dome-shaped members forming a microphone cell, a flange around the edges of each of said members and an electrode supported by the flange on each of said members.

5. In a telephone transmitter, a diaphragm comprising a pair of oppositely disposed conical shaped members,'a dome-shaped cup member clamped at its bottom 'to one of said conical members and in engagement with the other-oi said conical members-at the periphery of the cup, a housing for said diaphragm, means in said housing for clamping said diaphragm at its periphery, said diaphragm being flexible near its clamping periphery while remaining entirely rigid point near the periphery to its center and including said cup member.

6. In a transmitter, a conical diaphragm, a cup member, an eyelet for securing said cup member to said diaphragm and having an opening therein, a second cup memberdlsposed oppositely to said first cup member, electrodes secured to the edges of each oi said cup members, a flexible member extending between the faces of said electrodes, said cup members, electrodes, and flexible member completing the confines oi the microphone cell of said transmitter, a plugged opening in the bottom of said second cup member, carbon granular material filling said microphone cell and inserted through the opening of said second cup member, the opening in said eyelet being of a size to preclude the escape or the carbon granular material from said cell but permitting the escape of gases from said cell generdiaphragm secured to one end of said cell. granulated carbon material filling said cell, and means for preventing the escape of the carbon granules from said cell while at the same time permitting current through'said granules between said electrodes.

8. In a transmitter, a diaphragm comprising two conical members having their peripheries abutting, a microphone cell having one end attached to the apex of one of said conical members and the other end to an opening in the other conical member, and an electrode fastened to said microphone cell adjacentthe openingin said other conical member. x

9. A transmitter diaphragm comprising a pair .of conical members with their peripheries in abutment and their apexes extending in opposite directions and a portion of a microphone cell formed between said apexes.

10. A transmitter diaphragm comprising a pair of conical members having their peripheries in extending away-from one another to fonna hollow. space between said members, and a portion of a microphone cell disposed in said space.

the escape of gases generated by the passage of abutting relationship and their-concave surfaces 7 v comprising a pair of conical-shaped members having their peripheries in abutment and their concave surfaces extending away from each other, a portion of a microphone cell extending through an opening in the center of one of said conical members and attached to the apex of the other conical member, the apex of said other conical member and said microphone cell having an opening therein at their place of attachment, and an electrode secured to said microphone cell around the opening in said first conical member.

12. In a telephone transmitter, a pair of hemispherical shaped members having their open edges spaced opposite one another forming amicrophone cell, resistance material in said cell, means for insulating the walls of said cell from said resistance material, an electrode supported on the edges of each of said hemi-spherical members, flexible means extending between said electrodes, a diaphragm supporting one of said hencl spherical members at its base, and a rigid support for the other of said members.

13. In a telephone transmitter, a pair of hemlspherical shaped members arranged with their open edges spaced opposite one another and defining a microphone cell, a circular electrode supported on the edge of each of said members, resistance material in said cell, and means in sulating the cell walls of said members to confine the current flow through said resistance material and between said electrodes.

14. In a telephone transmitter, a microphone cell comprising a pair of cup-shaped members having their open edges facing each other and the bottoms of the cups extending in opposite directions, a ring-shaped electrode supported around the edge of each of said cup members, insulating material on the cell walls of said cup members for. confining the current flow between said electrodes, means for rigidly supporting one of said cup members, and a diaphragm for movably supporting the other cup member.

15. In a telephone transmitter, a microphone cell, a pair of electrodes in said cell, a diaphragm supporting one of said electrodes and movable therewith, means for rigidly supporting the other electrode, resistance material in said cell, means for confining the current flow through said cell between the electrodes and the resistance mate'rial, one end of said cell and said diaphragm having an opening therein for equalization of air pressure between the inside and outside of said. microphone cell.

16. In a. telephone transmitter, a microphone cell filled with resistance material, a diaphragm attached to one end of said cell, said cell and said diaphragm having openings in alignment therein extending from the inside to the outside of the cell to maintain the air pressure constant within the cell when in use, said opening being of a size to prevent the escape of the resistance material from said cell.

17. In a microphone cell containing resistance material and a diaphragm attached to one end of the cell, the provision of means for permitting the equalization of air pressure between the inside and the outside of the cell and diaphragm while preventing the escape of the resistance material therefrom.

18. In a telephone transmitter, a microphone cell, a diaphragm, a movable and a rigid electrode spaced apart in said cell, said movable electrode attached to said diaphragm and movable therewith under the action of sound waves impinging against said diaphragm, resistance material in said cell, an air passage extending from the inside of said cell to the side of the diaphragm receiving the sound waves, said air passage enabling the equalization of air pressure between the inside of the cell and the outside of said diaphragm to prevent the undesired movement of the movable electrode caused by the cell expansion.

19; In a telephone transmitter, a casing, a

conical diaphragm, a conical protective disc, a face plate having a conical surface, said protective disc arranged between said diaphragm and said face plate so that the conical surfaces of each element are in nested formation, said conical elements having extended flat surfaces parallel to eachother forming their clamping peripheries, said face plate securely clamping said protective disc and said diaphragm to said casing at said peripheri$ in such a manner that a narrow space is provided around their adjacent conical surfaces.

HAROLD C. PYE. 

